The blind rivet of the instant invention is an improvement on the composite rivet disclosed in U.S. Pat. No. 4,478,544 issued Oct. 23, 1984, for Composite Rivet, and assigned to the assignee hereof.
Carbon fiber reinforced materials are now widely used in the aircraft industry for airframe structural components. However, the use of carbon fiber reinforced resins in blind rivets has been limited by the difficulty of properly forming the blind head. One characteristic of composite materials utilizing carbon fibers is that the material often exhibits a rough finish after reforming due to protrusion or breakage of the carbon fibers. While the structural integrity of the material may not be compromised, the end product is abrasive and aesthetically unsatisfactory. When such material is employed in the reformable head of a blind rivet, the problem becomes acute since the blind head is exposed on the rear surface of the workpiece.
Another problem that must be addressed is that plastic cold flow of the rivet tends to reduce clamp-up forces on a workpiece. Thus, pretensioning of the rivet is highly desirable to offset ultimate relaxation of the tensile forces.
In the '544 patent, a high tensile strength, low modulus tubular sheath encapsulates the composite core of the rivet taught therein so as to prevent the protrusion of reinforcement fibers therefrom subsequent to blind head formation. However, the sheath relies upon the controlled and proper deformation of the underlying composite core to produce a blind head of a configuration suitable for use. It will be appreciated that the uncontrolled, nonuniform radial expansion of the composite core during blind head formation may displace the sheath so as to render its protection unavailing.
Additionally, the sheath as taught in the '544 patent provides no radially inward bias on the rivet's composite core in order to prevent cold flow thereof, nor does the sheath provide an axial bias against the anvil to pretension the rivet and, hence, minimize the reduction of clamp-up force upon plastic cold flow of the shear portion of the rivet's composite core.